While a hydrocarbon well is often no more than a foot in diameter, overall operations at an oilfield may be quite massive. The amount of manpower, expense, and equipment involved may be daunting when considering all that is involved in drilling, completing and managing a productive well. Indeed, for ease of management, the amount of footspace available and the desire to keep separate equipment in close proximity to one another may also be significant issues. This may be particularly true in the case of offshore operations, with footspace limited to a discernable platform.
Along these lines, in the area of coiled tubing assemblies, efforts have been made to minimize footspace requirements and provide a less cumbersome equipment set-up. For example, a conventional coiled tubing assembly includes an injector for driving up to several thousand feet of pipe from a reel and into a well at rates of between about an inch a minute to about 150 feet per minute. In addition to extensive depth, the coiled tubing may be driven through challenging well architecture such as highly deviated sections. Thus, power is generally obtained from a large diesel engine which powers a hydraulic pump that in turn drives the coiled tubing injector. This conventional set-up requires a large amount of footspace in addition to presenting management issues in terms of the presence of hydraulic oil and large, relatively stiff hoses. Indeed, mismanagement of the oil or failure of a hose may lead to failure of the entire assembly. Further, ensuring that the equipment is safely explosion-proofed presents its own set of challenges, particularly as emissions reduction requirements for the engine become more strict over time.
As indicated above, in light of the drawbacks to the conventional coiled tubing assembly set-up, efforts have been made to avoid use of the diesel engine or other hydraulic motors as a power source. For example, it has been proposed that the diesel engine be replaced with a 200 kW or so electric motor. This would eliminate the presence of hydraulic oil and hoses along with the failure modes associated with such aspects of internal combustion engines. Indeed, explosion proofing of an electric power source would be inherently improved over that of a diesel engine. Additionally, assuming the power supply is sufficient, use of a hydraulic pump may be eliminated and the amount of footspace required would be dramatically reduced.
Unfortunately, while well suited for operating at high rpm and power output, due to internal cooling limitations, an electric motor is not configured for operating at speeds that are dramatically variable. That is, as noted above, coiled tubing advancement may take place over a range of different speeds, from 150 feet per minute down to an inch a minute, for example. However, as the electric motor slows from directing a rate of 150 feet per minute to only an inch a minute, the cooling capacity of the motor also reduces. This is because the cooling system of an electric motor is tied to the rpm of the motor. Thus, even though speed is slowed, the current utilized is increased so as to ensure sufficient torque is employed throughout the operation. Therefore, the reduction in cooling capacity may lead to failure of the motor.
Efforts may be taken in order to address cooling issues with the electric motor when operating at a high torque/low speed ratio as noted above. For example, as opposed to relying solely on an internal cooling mechanism tied to motor rpm, liquid coolant may be introduced within the motor. However, this presents much of the same drawbacks as are found with hydraulic oil as described above. Furthermore, in the case of an electric motor which is configured to operate substantially friction free, the coolant introduces the inefficiency of a significant amount of drag.
Alternatively, electric motor cooling issues may be addressed by the introduction of added external cooling devices which may be coupled to the motor. However, this adds to the overall equipment size and footprint. Additionally, in order to ensure adequate safety and explosion proofing, an added level of complexity is introduced by the incorporation of flame traps between the external cooling devices and the motor. Thus, on the whole, options are available to help address heating issues of electric motors operating at variable and lower speeds. However, as such measures are undertaken, much of the potential benefit of employing an electric motor becomes lost. Indeed, as a practical matter, coiled tubing assemblies remain almost exclusively powered by diesel engines in spite of the smaller footprint and management advantages that are generally available from electric motors.